One of the dangers of ARTs is something called Ovarian Hyperstimulation Syndrome (OHSS). In order to make sure that this does not happen, my clinic monitored my hormone levels and follicle growth. The goal with ovulation induction therapy is that you want there to be two or three dominant follicles which grow much larger than the rest. The ones that grow large enough to be considered mature will release mature eggs once the HcG trigger injection is given. However, if the follicle growth is not properly monitored, they can be stimulated too quickly and many more follicles can reach mature size. This causes the ovaries to become very bulky and heavy and there is a risk of them twisting around upon themselves and cutting off the blood flow to the ovaries. There is also a risk of fluid being released from the cyst-like follicles and causing severe discomfort.
This is why I was required to return to my clinic every few days for careful monitoring of my cycle. This would consist of blood draw to check my estradiol (e2) level, and a follicle scan so the doctor could measure the dominant follicles. It was very much up to the doctor on call that morning to determine if the follicles had reached mature level. That, and my e2 level would determine when I was to receive my HcG trigger shot. I would watch the the doctor click his or her cursor on the u/s monitor at various places on each follicle to measure it. A follicle of 18mm or greater is considered mature.
Each day that I visited the clinic, one of the nurses would call me that afternoon to give me instructions for that evening's medication dose. These instructions were based on the e2 level and the size of the dominant follicles.